This document discusses the design of shafts that can experience twisting moments, bending moments, or a combination of both. It provides equations to determine the diameter of shafts subjected to twisting moments only based on the torque and material shear stress. Similarly, it gives equations for sizing shafts experiencing bending moments only based on the bending moment and material bending stress. For shafts with combined loads, it describes two failure theories and the resulting equivalent moment equations that can be used for design.